Calorimetric measurements serve to determine the amounts of heat turned over in chemical or physical processes as well as the specific heat of a substance. To perform this measurement, the substance or, more specifically, a substance sample, is heated in a calorimeter under controlled conditions, and the heat flow between a substance sample and a temperature control device (i.e. a device for heating and/or cooling) is observed by means of a thermoanalytical sensor. The heat flow is frequently determined by way of the temperature differences along the heat flow path between the sample and the temperature control device.
A calorimeter includes in general at least one measuring chamber with a thermoanalytical sensor on which at least one measurement position is arranged. The sensor is on the one hand thermally coupled to a temperature control device and on the other hand to a sample which is in contact with the sensor and/or to a reference. The temperature control device and/or the sensor is connected through suitable means to at least one controller unit.
Calorimeters can be used for example in the area of thermal analysis for the investigation of the most diverse kinds of substances. In general, a sample of the substance is set on a sample position formed on the sensor and is heated by means of a temperature control device, with the sample being held in most cases inside a special cup which can be closed up. The heat flow which occurs between the temperature control device and the sample is measured and evaluated. This can provide information about the structure and the composition of the substance, such as for example its heat capacity, phase transitions and oxidation stability. It is further possible to observe kinetic reactions and/or to make determinations of purity.
In calorimetric measurements that are to be performed with a high accuracy and over a large temperature range, the general procedure is to measure the heat flow of a sample in relation to the heat flow of a reference. Calorimetric measurements can be performed in calorimeters with separate measurement chambers as well as with a shared measurement chamber for at least one sample and at least one reference.
For accurate measurements with a high reproducibility, it is important that the sensor has a high mechanical, chemical as well as thermal stability. Sensors of the known state of the art often include a disk-shaped carrier with at least one thermocouple arrangement and at least one measurement position formed on the sensor. The thermocouple arrangement as well as the measurement position can be produced for example by means of thin-film technology or thick-film technology.
Sensors with thermocouples produced by thin-film technology that are part of a thermocouple arrangement are described for example in F. X. Eder, Arbeitsmethoden der Thermodynamik (Work Methods in the Field of Thermodynamics), Volume 2, Springer-Verlag 1983, page 240. Sensors produced by means of thin-film technology have the disadvantages that the production process is expensive, that the maximum thickness which can be realized is very small, and that they have in many cases a low mechanical and/or chemical endurance.
More resistant sensors can be produced by means of thick-film technology. A sensor on which one sample position and one reference position are formed and which has at least two thermocouple arrangements is disclosed in DE 39 16 311 C2. Sensors produced with thick-film technology can have several thick-film coating layers formed on a carrier substrate. The maximum overall thickness of the coating layers is about 100 μm, with the individual layers having a typical thickness between 5 and 20 μm. The individual coating layers can include electrical circuits such as for example thermocouple arrangements and are separated from each other by insulating layers. The layers are deposited by means of pastes and screen-printing techniques, and normally each printing step has to be followed by a firing process. As a result, the production process in particular for several coating layers is very time-consuming. The many sintering steps can have a detrimental effect on the structure and the properties of the coating materials involved and/or on the carrier substrate.
For the production of thermocouples, at least two different thermo-pastes are deposited on the carrier substrate which consists in most cases of a ceramic material. There can be a voltage difference between two thermocouples that are arranged at a distance from each other, through which a temperature difference can be determined. The thermocouples can be arranged in specific patterns that are prescribed for the deposition of the respective thick-film coatings.
With thin-film technology as well as with thick-film technology it is possible to implement substantially two-dimensional thermocouple arrangements which detect temperature changes within the layer of the sensor that contains thermocouples. The thermocouples are normally disposed in a substantially horizontal layer of the sensor, either on the sensor surface itself or near the latter.
However, the heat flow in the sensor is not confined within a layer but propagates in three dimensions in the entire sensor. Thus, only a part of the heat flow can be measured with a substantially two-dimensional thermocouple arrangement produced with thin- or thick-film technology, so that the result of the measurement carries a corresponding uncertainty.